The present invention relates to a transmitted X-ray data acquisition system and an X-ray computed tomography system. More specifically, the present invention relates to a system for acquiring transmitted X-ray data based on which an X-ray tomographic image is produced, and an X-ray computed tomography system for producing the tomographic image on the basis of the acquired transmitted X-ray data.
In X-ray computed tomography systems, an X-ray irradiating/detecting device acquires transmitted X-ray data that represents X-rays transmitted by an object of imaging, and a tomographic image of the object is produced (reconstructed) based on the transmitted X-ray data. The X-ray irradiator irradiates an X-ray beam that spreads (has a width large enough) to cover a tomographic layer of the object or a radiographic section thereof and has a thickness in a direction perpendicular to the direction of the layer or section. The X-ray detector is a multi-channel X-ray detector that has a plurality of X-ray detecting elements arrayed and detects the X-ray beam. The X-ray irradiating/detecting device is rotated about the object (in order to scan the object) in order to acquire projection data, that is, transmitted X-ray data in a plurality of directions determined for respective views around the object. A tomographic image is then reconstructed based on the acquired transmitted X-ray data items by means of a computer.
In order to produce a high-quality tomographic image, the conditions for X-irradiation are adjusted depending on an object. When an object exhibits a larger absorption dose, the object is imaged with X-rays of a larger radiation dose. The radiation dose of X-rays is determined with a product of a tube current by a conduction time, that is, a milliampere-per-second (mAs) value.
An image standard deviation (image SD) is adopted as one of indices indicating the quality of a reconstructed image. The image SD exhibits a strong correlation with a projection area in an object whose projection is created as long as the product of a tube current flowing through an X-ray tube by a conduction time remains constant. In order to produce a tomographic image exhibiting a proper image SD, the product of a tube current by a conduction time is automatically adjusted depending on the projection area. For the automatic adjustment of the product of a tube current by a conduction time, X-ray fluoroscopy is performed in advance in order to determine a projection area, and the product of a tube current by a conduction time is appropriately determined depending on the size of the projection area.
Sections of a human body that is an object of imaging are generally oval. An X-ray absorption dose differs between the major-axis directions of the section and the minor-axis directions thereof, the product of a tube current by a conduction time determined as mentioned above is corrected based on an oval ratio, that is, a ratio of the major axis to the minor axis. Due to the correction, the larger the oval ratio, the larger the product of a tube current by a conduction time.
In order to measure the lengths of the major and minor axes, X-ray fluoroscopy is performed by irradiating X-rays to the object in the anteroposterior or posteroanterior direction (at 0° or 180° to an X-ray tube) and in a transverse direction (at 90° or 270° thereto). The 0° or 180° direction corresponds to the anteroposterior or posteroanterior direction, and the 90° or 270° direction corresponds to the transverse direction. The lengths of lines passing the center of a projection created with X-rays irradiated in each of the anteroposterior or posteroanterior direction and the transverse direction are measured. The longer one of the lines is regarded as the major axis and the shorter one is regarded as the minor axis. Consequently, the oval ratio is calculated.
The X-ray irradiating/detecting device is rotated along a helical trajectory, whereby helical scanning is achieved. During the helical scanning, an object is scanned continuously over a predetermined length in a body-axis direction thereof. Therefore, a plurality of tomographic images representing sections of the object that extend at different slicing positions on the body axis can be reconstructed based on acquired transmitted X-ray data.
During helical scanning, the positions on a body axis which X-rays pass vary continuously during rotation of the X-ray irradiating/detecting device. Therefore, the product of a tube current by a conduction time is adjusted depending on a projection area in an object that extends at an intermediate scanning position. What is referred to as the intermediate scanning position is a position on the body axis which X-rays pass at an intermediate time point within one rotation.
A radiation dose dependent on the product of a tube current by a conduction time that is corrected based on an oval ratio is retained at the same value during one rotation of the X-ray irradiating/detecting device about an object. The dose is appropriate for the portions of a section containing the major axis thereof but is excessive to the portions thereof containing the minor axis thereof. Consequently, a total dose is too large and excessive X-rays are irradiated.
Assume that helical scanning is performed by moving the X-ray irradiating/detecting device by a long distance along a body axis during one rotation, that is, helical scanning is performed by moving the X-ray irradiating/detecting device in units of a large pitch. In this case, a variation of a projection area in an object whose projection is created during one rotation tends to increase. The product of a tube current by a conduction time determined relative to the intermediate scanning position is not always appropriate for the other positions.